A nicely priced E92 M3 popped up locally. I received a copy of the Carfax today and it shows airbag deployment.

I've known Carfax to report incorrectly in the past. I've seen it where a car was rearended and the airbags in the other vehicle deployed, but Carfax reported that the airbags in the front vehicle deployed.

The current owner of the vehicle is stating that there are sensors in the back that deploy airbags at impact of 10mph or greater? I find that difficult to believe, but I'm not an expert by any means.

Can anyone on here clarify what an e92 M3 should do if it is rearended?

There are safety measures that can be deployed at any time the key is on and there is an impact that requires it. As for the airbag system logic, that I doubt anyone truly knows/understands it. It's all based on the changes in lateral acceleration the car experiences at the moment of impact. ~'09- production have active head restraints that are meant for head and neck injury prevention during rear end collisions. Those, along with the seat belt tensioners, are usually deployed in a rear end collisions/impacts. If you want to know exactly what happened, contact the body shop that performed the repairs. They may be able to tell you the severity of the collision, or at least the replacement parts installed.